Big sur problem -- doesn't open top window when switching applications (Firefox)

Macro worked fine before, opening URLs in a draft Mail message in Firefox simply by activating Firefox, where the top window I've been using is the one I want the URLs to open in. But now with BS (Big Sur), it picks a different window in Firefox, one that I have set to be much smaller, and opens all the URLs in that window. I assume it is picking the first window in Firefox based on alpha sorting of windows' URLs? Who knows. I don't know how to make KM pick the most recent (top) window in Firefox when moving from Mail.

Further note: Looks like BS opens the most recently added window in Firefox, being the last one in the list of Windows when KM switches from Mail to Firefox with Activate Last Application action. I might be able to train myself to keep my "working" Firefox window at the bottom, by opening it last, and nothing later, but I'm too old to learn that, probably.